function img = extract(E)
%提取秘密图像
[H, M] = size(E);
I = zeros(H/2, M/2);
for i = 1: 2: H
    for j = 1: 2: M
        I((i + 1)/2, (j + 1)/2) = E(i, j);
    end
end
img = uint8(I);
end